Compositions of oil-based biodegradable drilling fluids and process for drilling oil and gas wells

Abstract

Compositions of oil-based biodegradable drilling fluids are described, the compositions comprising an oil phase composed by methyl esters of soybean oil fatty acids, an aqueous phase finely dispersed in the oil phase by specific emulsifiers and several other additives having specific functions for providing suitable performance to the finished drilling fluid composition. The process to manufacture the inverted emulsion and the drilling process that employs the compositions of the invention are described too.

1. Compositions of biodegradable oil-based drilling fluids, the compositions being inverted emulsions specifically formulated to be compatible with the oil phase, wherein such compositions comprise, on a weight basis:
 a) of from 48 to 75% of a continuous oil phase essentially made up of fatty acid methyl esters of natural source oils; 
 b) of from 22 to 42% of an aqueous phase compounded with a finely dispersed alkali metal halide; 
 c) of from 2 to 8% a of at least, one emulsifying agent consisting essentially of fatty acids soaps suitable to yield an oil-in-water emulsion; 
 d) of from 0.5 to 1.0% a of a viscosifying agent of the organophilic clays type; 
 e) of from 0.1 to 0.5% of, at least, one fluid loss polymeric additive, from the class of polyacrylates; 
 f) of from 0.1 to 0.5% of, at least, one antioxidant additive selected from the group consisting of butylated-hydroxy-anisole (BHA), butylated-hydroxy-toluene (BHT), tert-butyl-hydroquinone (TBHQ), and 2,6, di-tert-butyl-p-cresol (DBPC); 
 g) of from 1 to 3% a of a metallic oxide able to cause an alkaline reaction in order to produce in situ the fatty acids soaps and provide an alkaline reserve to the compositions. 
 
     
     
       2. The compositions of  claim 1 , wherein the compositions additionally comprise a pour point depressant additive. 
     
     
       3. The compositions of  claim 2 , wherein the pour point depressant additive is d-limonene, in concentrations of from 0.1 to 10% by weight. 
     
     
       4. The compositions of  claim 1 , wherein the compositions additionally comprise a barium salt as weighting agent. 
     
     
       5. The compositions of  claim 4 , wherein the barium salt is barium sulfate in concentration of from 0.1 to 25% by weight. 
     
     
       6. The compositions of  claim 1 , wherein the compositions additionally comprise a sealing agent. 
     
     
       7. The compositions of  claim 6 , wherein the sealing agent is calcium carbonate in a concentration of from 6 to 14% by weight. 
     
     
       8. The compositions of  claim 1 , wherein the natural source oil is of animal origin. 
     
     
       9. The compositions of  claim 1 , wherein the natural source oil is of vegetable origin. 
     
     
       10. The compositions of  claim 9 , wherein the vegetable oils used as fatty acid source to synthesize the said methyl esters employed as the continuous oil phase of the emulsion are soybean, castor, corn, cottonseed, rapeseed and peanuts oil, or admixtures of same, in any amount. 
     
     
       11. The compositions of  claim 10 , wherein the preferred source of fatty acid to synthesize the methyl esters is soybean oil. 
     
     
       12. The compositions of  claim 11 , wherein the amount of soybean oil methyl esters in the continuous oil phase a) is 55 to 65% by weight. 
     
     
       13. The compositions of  claim 1 , wherein the aqueous phase, having a finely dispersed alkali metal halide b) is made up of from 26 to 30% by weight of a saturated solution of sodium chloride. 
     
     
       14. The compositions of  claim 1 , wherein component c) further comprises a mixture of amines and amides, prepared from crude soybean oil. 
     
     
       15. The compositions of  claim 14 , wherein component c) further includes free fatty acids to allow “in situ” soap formation. 
     
     
       16. The compositions of  claim 14 , wherein component c) is present in concentration of from 3 to 6% by weight. 
     
     
       17. The compositions of  claim 1 , wherein the fluid loss additive of component e) is a copolymer of succinimide polyacrylate. 
     
     
       18. The compositions of  claim 1 , wherein component e) comprises at least two fluid loss additives, one of them being an auxiliary fluid loss additive. 
     
     
       19. The compositions of  claim 18 , wherein the auxiliary fluid loss additive is sodium polyacrylate in concentration of from 0.1 to 0.5% by weight. 
     
     
       20. The compositions of  claim 1 , wherein the compositions additionally comprise an auxiliary antioxidant additive of component f). 
     
     
       21. The compositions of  claim 20 , wherein said auxiliary antioxidant additive is citric acid in concentration of from 0.5 to 1% by weight. 
     
     
       22. The compositions of  claim 1 , wherein the “in situ” soap formation is obtained by adding the metallic oxides of component g), during the emulsion manufacturing. 
     
     
       23. The compositions of  claim 22 , wherein the metallic oxide of component g) is magnesium oxide. 
     
     
       24. The compositions of  claim 22 , wherein during the emulsion manufacturing, magnesium oxide, when added in excess to that required to form the “in situ” soaps, is in charge to promote the alkaline reserve necessary to keep the emulsion stability. 
     
     
       25. The compositions of  claim 23 , wherein the concentration of magnesium oxide is of from 1 to 3% a by weight. 
     
     
       26. The compositions of  claim 25 , wherein the control of the excess magnesium oxide is effected by adding citric acid in concentrations of from 0.1 to 1.5% a by weight. 
     
     
       27. The compositions of  claim 1 , wherein the pH of the emulsion is kept in the range of from 9 to 10. 
     
     
       28. The compositions of  claim 1 , wherein the compositions are prepared according to the following steps:
 a) pouring into a suitable mixer a desired amount of the esters and, if necessary, d-limonene; 
 b) adding, under mixing, the whole amount of emulsifier; 
 c) adding half of the required amount of magnesium oxide; 
 d) adding a desired amount of brine while keeping the mixer working for the period of time necessary to homogenize the mixture, the operation lasting as long as two hours; 
 e) adding the other half of the required amount of magnesium oxide; 
 f) adding a desired amount of the main fluid loss additive; and 
 g) adding a desired amount of the viscosifying agent. 
 
     
     
       29. The compositions of  claim 28 , wherein the components of such compositions are combined and balanced in order to keep the emulsion stabilized and with a gel strength enough to keep in suspension the solids produced in the drilling, even though in the presence of methyl esters of a lower relative viscosity and more susceptible to deteriorate by hydrolysis than other oil phase esters. 
     
     
       30. The compositions prepared in accordance with  claim 28 , wherein the compositions have a dynamic character, meaning that during the operation component amounts are continuously adjusted, to adapt the properties of the emulsion to the needs of the drilling operations. 
     
     
       31. The compositions prepared in accordance with  claim 28 , wherein the compositions additionally comprise at least one of a weighting agent and a sealing agent, and wherein the compositions are prepared according to the further steps of
 h) collecting and analyzing in the laboratory a sample of the mixture and, if it matches the specification, transferring it immediately to tugboat tanks which will deliver the mixture to a designed platform; and 
 i) adding to the composition the sealing and weighting agents only in mixing tanks of the platform. 
 
     
     
       32. The compositions prepared in accordance with  claim 28 , wherein d-limonene is added on shore to the compositions, together with the esters, in the mixers of a mixing plant. 
     
     
       33. A process for drilling oil and gas wells, with the aid of the compositions formulated according to  claim 1 , wherein the process comprises the following steps:
 a) specifying the range of physical-chemical characteristics of the drilling fluid including at least one of viscosity and density, starting from data acquired by mathematical and mechanical simulators which estimate well drilling conditions for each well to be drilled; 
 b) preparing a desired amount of fluid composition, without weighting and sealing agents, which will be added to the tanks at a local drilling operation in order to provide the fluid composition with the physical-chemical characteristics determined in a); 
 c) pumping said fluid composition prepared in b) to the tanks at the local drilling operation and adding and mixing a weighting agent and, eventually, sealing agent; 
 d) starting the drilling operation using said fluid composition; 
 e) gathering and recovering the non-utilized fluid composition as prepared in b) for future operations. 
 
     
     
       34. The compositions prepared in accordance with  claim 31 , wherein the weighting and sealing agents are added to the compositions in tanks of a local drilling operation, just before using the compositions in the operation. 
     
     
       35. The compositions of  claim 1 , wherein the antioxidant additive is 2,6, di-tert-butyl-p-cresol (DBPC).